1. Field of the Invention
This invention relates to methods and apparatus for predicting the cycle-down characteristics of the photoreceptor. More particularly, through the use of historical values and actual measured values, the characteristics of the decaying charge potential on the photoreceptor can be predicted for the next cycle.
2. Description of the Related Art
In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged. A photoreceptor belt has a typical charge retentive surface. A light pattern formed from the original image to be reproduced selectively discharges the charge on the photoreceptor. The resulting pattern, a combination of charged and discharged areas on the photoreceptor, form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image area by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being produced. The toner image may then be transferred to a substrate (e.g., paper), and the toner is fused onto the substrate by passing through a fuser. At this point the image is affixed to the substrate and is ejected from the machine to the holding tray. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly.
In order for material to be useful as a photoreceptor, it must first be able to accept and maintain charges to relatively high voltage levels. Several hundred volts is a common charge level. Ideally, once charged, the photoreceptor would maintain a constant voltage until exposure to light. The voltage drops to a level near zero after the photoreceptor is exposed to light.
All known photoreceptor materials exhibit non ideal behavior. Photoreceptors cannot maintain a constant non-zero initial voltage level due to several physical mechanisms that tend to dissipate charge. The decay of the unexposed voltage level on a photoreceptor is called the dark development potential (V.sub.ddp). It is important in developing the contrast characteristics of the latent image. The dark development potential can be expressed as a function of time after the initial charging of the photoreceptor.
In order to use a photoreceptor in a printer or copier, the decay of the dark development potential V.sub.ddp needs to be controlled. Therefore, a prediction of the dark development potential V.sub.ddp is made after the initial charge is induced on the photoreceptor. If the decay of the dark development potential V.sub.ddp were a simple and repeatable function of time after initial charge, then the problem of predicting the dark development potential V.sub.ddp level would be trivial. However, dark development potential decay typically is not a simple function because it is dependent upon several parameters some of which are neither constant nor easily measured. The first parameter is the rest time that has occurred since the last use of the photoreceptor. For example, the ability of the photoreceptor to maintain the charge will be different if the last time the photocopier machine was operating was ten minutes ago, last night, or a week ago. The second parameter is the number of copies that were made. For example, if only one copy was made, the charging of the photoreceptor belt will be different than if a thousand copies were made. Third, humidity and temperature may have a small effect on the characteristics of retaining a charge on the photoreceptor belt 10.
Predicting the rate of decay is the main difficulty when dealing with the dark development potential V.sub.ddp. If a photoreceptor has been idle for a long period, it tends to retain charge well. In contrast, if a photoreceptor is being repeatedly charged and discharged, such as when several prints are made successively, a degradation in charge retention is noticeable. Therefore, the rate of decay of the dark development potential V.sub.ddp tends to increase with every successive use of the photoreceptor.
The change in the decay rate of the dark development potential V.sub.ddp during successive print cycles is called cycle-down. If successive use continues long enough, cycle-down ends when a steady state condition is reached. In this state, the rate of decay stabilizes or at least changes so slowly that it can be considered reasonably constant. After steady state is reached, the simplest method is to record a sample charging level, a resulting dark developing potential V.sub.ddp, and the sensitivity of that sample charging level to changes in the charging conditions. The sensitivity is determined during steady state by comparing dark developing potential V.sub.ddp measurements under different charging conditions. This method is essentially a linearization of the function in which the data pair (charging conditions, resulting dark developing potential of V.sub.ddp) becomes the operating point. The sensitivity becomes the slope of linearization.
In the prior art, a method to predict the cycled own effects required a technician to measure the steady state V.sub.ddp level during an off-line setup procedure. This information would be entered into a non-volatile RAM. At the beginning of each print run, the controller would take the difference between the measured dark developing potential V.sub.ddp and a predetermined steady state dark developing level, to estimate the total cycle-down that will occur. The predetermined fraction of a total cycle-down is assume to occur on each cycle. The problems with this method are: first, a technician is required in the beginning to establish the benchmark levels; second, there are no allowances for inevitable changes in the behavior of the photoreceptor belts over time; and third, there are no allowances for the random variability of behavior in photoreceptors of the same type, or even for the photoreceptor being used.